1. Field of the Invention
The present invention relates to a piezoelectric ceramic material and, more particularly, to a piezoelectric ceramic material having characteristic crystal grain conditions.
2. Background Art
Piezoelectric ceramics are widely used in piezoelectric devices such as resonators, filters, surface acoustic wave filters, IR sensors, ultrasound sensors, piezoelectric buzzers, and piezoelectric actuators. Such piezoelectric ceramics predominantly comprise lead titanate zirconate ceramic material. Typically, the piezoelectric ceramics are produced by mixing in predetermined compositional proportions lead oxide, titanium oxide, and zirconium oxide serving as predominant components; shaping to form a compact; and firing the compact.
Japanese Patent Application Laid-Open (kokai) Nos. 58-204579, 4-305057, and 5-114308 disclose, among such piezoelectric ceramics, some ceramics incorporating a glass component.
Specifically, Japanese Patent Application Laid-Open (kokai) No. 58-204579 discloses piezoelectric ceramics incorporating a silicate glass compound, a soda glass compound, or a lead compound in an amount of 1-30 wt. %.
Japanese Patent Application Laid-Open (kokai) No. 4-305057 discloses piezoelectric ceramics in which a glass component is crystallized to form a crystal grain boundary phase. Japanese Patent Application Laid-Open (kokai) No. 5-114308 discloses ceramics in which a ferroelectric ceramic is dispersed in a glass matrix.
However, such piezoelectric ceramics contain glass as a matrix component, and the glass is incorporated into a crystal grain boundary phase. Therefore, the ceramics have poor mechanical strength, poor resistance to chemicals such as acid and alkali, and poor moisture resistance, and reliability of the ceramics must be improved.
In view of the foregoing, the present inventors have conducted extensive studies, and have found that a piezoelectric ceramic material having specific grain boundary conditions solves the aforementioned drawbacks.
Accordingly, the present invention provides a piezoelectric ceramic material containing lead oxide, wherein a closed region observed under a transmission electron microscope and defined by a plurality of crystal grains which constitute the sintered piezoelectric ceramic material and are adjacent to one another (hereinafter simply referred to as a xe2x80x9cclosed regionxe2x80x9d) predominantly comprises a glass phase including a network-forming oxide and no glass phase is observed at grain boundaries outside the closed region under a transmission electron microscope.
Preferably, in the piezoelectric ceramic material of the present invention, the total weight of the network-forming oxides relative to the total weight of the grains is about 100 to 700 ppm.
The present invention also provides an electronic part produced from the piezoelectric ceramic material.